In Sherlock season 1 episode 1 the serial killer asks Sherlock to take one of two bottles. 1 bottle is ok other is bad (poisonous). Whatever bottle Sherlock takes the serial killer takes the other one. That is same way the serial killer murdered his other victims.

But the episode doesn't say how exactly serial killer achieved this. Before Sherlock can take one of the bottles the serial killer is murdered.

Does anybody know how the serial killer achieved this?

  • Has it been asked why would the victims even want to take the pill? The lady MP was certainly not taking a taxi.
    – Old Geezer
    Aug 9, 2018 at 13:24

7 Answers 7


The cabbie is indeed shown to be terminally ill and wants to raise money for his family. However, this only explains why he resorts to killing people (=earn money fast). Motives of Moriarty are part of the bigger overall story arc in the Sherlock Holmes universe as Moriarty is the arch-nemesis of Sherlock.

The poison pill trick is not random. The cabbie is skilled in reading and manipulating people. He has analyzed what drives his victims and acts accordingly. He knew Sherlock would get in his cab instead of calling the police. The poison pill trick is his way of showing he has control through his skill. His victims can make the choice but as they are choosing, the cabbie manipulates them. He knows which of the bottles is poisonous and will manipulate his victims into taking the bad bottle. If they take the good one, he will convince them to change their mind. This shows of how good the cabbie is in reading people, predicting their train of thought and knowing what to say to mislead them. It takes great skill to manipulate people that know you are trying to kill them.

His confrontation with Sherlock is a battle in the understanding of human behavior. It is a match to decide who can better see through the other's act. They have layers and layers of bluff/misdirection in all the small details of their actions.

  • This again is a theoretical answer. Choosing from the 2 bottles is pure chance. No matter how good he is at manipulating people it is still a huge chance. What happens if victims chooses to stick to their choice and has in fact chosen the good bottle. Its very unlikely way to kill 3 victim. I just believe there had to be some trick involved that he used. He did say he would win every time he tried; so it just can't be a matter of chance.
    – minusSeven
    Mar 13, 2013 at 8:52
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    @minusSeven You seem to underestimate the effect that his mind manipulation can have for raising his chances. It may not have worked with Sherlock, who may have realized that sticking to the 50% of his initial choice is better than messing with his mind plays. But the other victims maybe really thought they could beat him or were easier to manipulate. Probably not 100% for the cabbie, but definitely better than 50% and maybe sufficient for 3 easy victims. If you really expected a "non-theoretical" actual (physical) trick, then no, there is none.
    – Napoleon Wilson
    Mar 13, 2013 at 8:57
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    @minusSeven "He did say he would win every time he tried" - And you believe him to say the ultimate 100% thruth, especially considering the mind manipulation context? Congratulations for taking the wrong pill. ;)
    – Napoleon Wilson
    Mar 13, 2013 at 8:57
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    @minusSeven: It is clearly shown in the episode that the cabbie is skilled at predicting human behavior. I think it is highly unlikely that the writers would portray him as being 'Sherlock using his powers for evil' and would then use a cheap trick to explain his success. It's also unlikely that if his victims would choose the correct bottle, he would simply take the poison. If you're really looking for a trick : he promises to take whichever bottle is left but just shoots his victims when he would lose.
    – Origin
    Mar 13, 2013 at 9:31
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    or maybe both bottles are poisonous and he has an antidote. or maybe he would force feed his victims the poison when he loses. There is nothing forcing him to honour his "I take what's left". However, without an actual scene from one of the murders, all answers are speculative. I just believe my posted answer is best in line with the whole episode and the characters portrayal.
    – Origin
    Mar 13, 2013 at 9:35

The killer was terminally ill with a brain aneurism and could die at any moment. He was using the 'scheme' to raise money. He was earning money from Moriarty, who was essentially sponsoring the cabbie to murder people in this manner. Moriarty's motive is unclear, perhaps just to cause chaos, but more likely to get his name in front of Sherlock.

Essentially the cabbie had nothing to lose - so it was purely random which pill was poison. On each occasion he had a 50% chance of winning. As the only repeat player, the odds are that he would die within a few rounds, but a fairly good chance he would kill one or two in the process - raising money for his family. He viewed himself as being expendable, likely to die at any moment anyway - an intelligent man, he viewed it as a risk worth taking.

[Alternative (not very serious theory): Perhaps he has built up an immunity to Iocaine powder?]

  • Hmmm. That's an interesting explanation. Mar 12, 2013 at 18:03
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    the guy killed 3 people before Sherlock. That makes the chances about 12%. And he did say he would win every time he tried. There had to be some trick involved.
    – minusSeven
    Mar 12, 2013 at 18:08
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    @minusSeven: I agree 12% is small odds, but far from being impossible. However each event is a separate 'roll of the dice'. Once he survive he might as well do it again - he has a 50% chance of wining again. I think the 'win every time' could be part of the 'script' he's been given from Moriarty to entice Sherlock.
    – iandotkelly
    Mar 12, 2013 at 19:09
  • And I always thought he was some kind of poker genius able to read other peoples' minds. But maybe this whole "I can play everybody's mind to always win the correct bottle"-show was really just to distract Sherlock.
    – Napoleon Wilson
    Mar 13, 2013 at 8:51
  • He is also able to apply some manipulation should the wrong bottle be chosen by the victim. This must skew the odds a little (but not much) in his favor.
    – iandotkelly
    Mar 13, 2013 at 15:20

Both pills have poison. You see but you do not observe!

The cabbie opens the bottle and holds the pill in his left hand. He talks and distracts Holmes. Just before he is shot you see him with a pill in his right hand. He had a second, non-poison pill, all the time. He did this with all the victims. None had a chance, the game was rigged.

At time 1:19:36 the pill is in his left hand.

At 1:19:36 the pill is in his left hand

At time 1:19:54 the pill is in his right hand.

At time 1:19:54 the pill is in his right hand

  • 5
    If you can add some evidence like screenshots and such this would be a much more complete answer. Right now it seems more like a rambling opinion without proper structure.
    – Tablemaker
    Aug 23, 2013 at 12:01
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    Interesting answer. Just watched this and the pill does actually switch hands between cuts but it appears more likely to me to be a simple filming/editing continuity error. I didn't see any frame with both hands visible at once. May 16, 2014 at 5:54
  • While theres no evidence, this seems like the most reasonable answer that is not "it's purely up to chance"
    – Nate D
    Dec 12, 2017 at 17:21
  • Correct! Perhaps this is the reason why cabbie remained silent even during his last moments of life when Holmes asked him which pill was poisonous. Mar 28, 2019 at 9:47

There's another explanation although without evidence of course.

Neither of the pills were poisonous. The poison was in a glass of water he would offer the victims to wash it down with. He'd dry swallow the pill. In my opinion it's one of the better answers.

However everything is entirely speculation so it could easily be wrong.

  • 1
    No. no. He spent the last 3 years building up an immunity to Iocaine Powder...or was that a different movie :)
    – MrInfinity
    Oct 29, 2018 at 4:05

Could be the pills are his meds. If he takes them it does nothing (they're prescribed for a reason), but when someone else takes them they die.

  • That is a surprisingly good answer.
    – Joshua
    Dec 7, 2016 at 22:21
  • Interesting angle. Seems more probable to find drug where you can simply take anti dote beforehand.
    – arberg
    Apr 8, 2020 at 21:16

I heard my favorite theory on tumblr and expanded on it. Neither pill was poison, and the water he gave them was. He swallowed his dry. This explanation has no evidence in the episode to support it, but it still fits the events and I like it a lot.

One problem that I noticed is that there is no water in the final scene where they are both about to take the pills. One way to explain this is that the cabbie's sponsor Moriarty (who has been described as a 'fan' of Sherlock) did not want Sherlock dead. If neither of them used poison water then neither one would die. Presumably, the cabbie would fake his own death, let Sherlock think he had chosen right, and then who knows?

  • I was with you right up until I read "fake your own death" this is Sherlock we're talking about!
    – Liath
    Aug 1, 2013 at 10:16

This is covered in the episode, Holmes mentions it to the cabbie and the cabbie replies something like:

Four games and four wins, that's not random

If you want an excellent, explained example of this in action - watch The Princess Bride:


The man in black's intelligence test. He provides two vials, one of which is poisoned.

Vizzini explains his chain of thought thus expressing how he feels that the placement of the poisons is not random but is a psychological trick. In the same way the taxi driver feels that there is psychology in the way he plays his 'game' and that it is not random - whether either if them are right or wrong is not the point, the point is that they feel there is more to it than random chance

Note: the twist in the Princess Bride's version about immunities is not relevant to the Holmes's version. I only included this as it explained the chain of thought as to which glass should be drunk from.

  • Can't really watch at work but why does the Princess Bride explain this situation?
    – Tablemaker
    Mar 13, 2013 at 13:16
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    The man in black's intelligence test. He provides two vials, one of which is poisoned. The Sicilian talks through his reasoning as to whether he should take the vial closest or furthest depending on where he thinks the Man in Black would put it.
    – Stefan
    Mar 13, 2013 at 13:24
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    Most likely cause the answer is shorter than others and maybe not fully read through. I will say this though, if you include that summary of the video with your answer that will help your answer :D
    – Tablemaker
    Mar 13, 2013 at 17:04
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    @minusSeven: that's speculative, there is nothing in the episode that hints at any immunity, they say nothing about the cabbie being immune
    – Origin
    Mar 15, 2013 at 12:36
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    Vizzini's 'logic' is not meant to be taken seriously (e.g. "the poison comes from Australia, and Australia is entirely people with criminals, ergo...), so I'm not sure how this tongue-in-cheek scene from The Princess Bride is supposed to explain what happens in 'Sherlock Holmes'.
    – Oliver_C
    Mar 16, 2013 at 16:39

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