How far would you go to be with the one you love? Would you donate your organs to them? Would you take a bullet for them? Would you take “together forever” literally, in body and mind, bound eternally by bone and flesh?
As their relationship continues to stagnate with no progression, Tim (Dave Franco) and Millie (Alison Brie) move out of the city and to the countryside. An elementary school teacher, Millie hopes to get married to Tim, but her boyfriend is too traumatised by the bizarre death of his parents.
Not long afterwards, the couple decide to take a hike in the forest around their new house. As a rainstorm hits the area, they inevitably get lost before falling into an underground cave. Seeking refuge until the downpour is over, they are forced to spend the night and drink the water from a pool inside.
In the following days after climbing out, Tim and then Millie begin experiencing bodily and psychological changes.
