I thought it might help to explain just the mechanics of the events on board the Aeolus a bit to clear up what's happening. I am loosely following this video analysis by Peng Yang, though I think I simplified the explanation a bit.
The two key aspects of the main plot are these: First off, there are two separate groups of people arriving on the ship. I call them the left-movers and the right-movers, respectively, named after the direction in which the Aeolus is shown moving when they board:
Left-movers (left image, 0:21:25), right-movers (right image, 0:43:00).
I use the prefixes L- and R- henceforth to refer to the two kinds (or "chiralities") of groups. Groups of survivors keep arriving in a never-ending sequence: L, R, L, R, ...
The second key point is that at any given time, there are three versions of Jess on the ship. This immediately implies that the action falls into two distinct phases:
- Phase A: 2 × L-Jess, 1 × R-Jess
- Phase B: 1 × L-Jess, 2 × R-Jess
The action consists of alternating phases A, B, A, B, ...
Each phase ends with the "departure" of one Jess and the arrival of a new group of the opposite chirality:
- Phase A ends when an L-Jess is forced overboard by the other L-Jess; a new R-group arrives.
- Phase B ends when an R-Jess is hacked to death by the other R-Jess; a new L-group arrives.
The fact that three versions of Jess exist simultaneously means that each Jess (of each chirality) experiences interaction with her own future (or past) self, as well as interaction with a different Jess.
Now, as we track each Jess's life cycle aboard the ship, we can assign "age" labels to her: L1, L2, L3; R1, R2; R3. Age 1 is the "young" age of the fresh arrival, age 2 is the "only" age (explained presently), and age 3 is the "old" age of immanent departure. During each phase we have thus:
- Phase A: L1-Jess, R2-Jess, L3-Jess: A young and an old L-Jess, and an only R-Jess
- Phase B: R1-Jess, L2-Jess, R3-Jess: A young and an old R-Jess, and an only L-Jess
At the end of each phase, the 3-Jess departs (at the hands of the 1-Jess of the same chirality), and a new group of the opposite chirality boards the ship. At that point, we increment the age labels and insert the new 1-Jess from the newly arrived group. So the progression of Jesses is:
[L1, R2, L3], [R1, L2, R3], [L1, R2, L3], [R1, L2, R3], ...
Or, in a 2D picture:
Arrival ... L1 R1 L1 R1 ...
\ \ \ \
\ \ \ \
... R2 L2 R2 L2 ...
\ \ \ \
\ \ \ \
Departure ... L3 R3 L3 R3 ...
--------------- time --------------->
Phase ... A B A B ...
It is also worth noting that except Jess, no other character meets any other version of themselves. Everyone except Jess gets killed in their "young" phase (by 3-Jess).
By the way, there are some nice visual clues in the film (and in the poster) regarding the presence of three Jesses:
There are some open questions:
I don't see evidence regarding whether the two different chiralities of Jess are related. Peng Yang suggests that the R-Jess is the continuation of the L3-Jess that falls overboard, and that the Jess that goes to the harbour boards the Aeolus with full memories as R1, whereas after the death of R3 the loop restarts with L1 waking up from the dream with no memories. I'm not sure if that view is warranted; both Jesses don't seem surprised when they spot the Aeolus for the first time, yet both Jesses also show signs of surprise at some of the events (which would be at odds with having foreknowledge of the events).
Another matter is the question how the entire "afterlife" started. I think it's clear that Jess's son's death has something to do with it, but whether she originally killed him at their home or in a car accident later isn't so clear. I like the idea that real-world Jess has blood on her dress (not paint), and that it's the boy in the bag, and it's Jess that dies in the car accident at 8:17, whereas the boy is already dead at that point.