Why does Muri put her trust in Dan?
After the rescue mission in Miami, Dan comes face-to-face with his daughter Muri (Yvonne Strahovski), all grown up and leading the resistance against the aliens. She reveals to Dan that, in her timeline, he eventually separated from Emmy and left the family, falling into a deep depression that ultimately leads to his death in a car accident. Naturally, this resulted in Muri harboring a great deal of resentment for the man he became.
She tells him that she plucked him from the past because she needed the father she knew before he went off the deep end, when he was the hero she remembers adoring as a child. Shocked by the news that he’s essentially destined to abandon his family, Dan commits to helping Muri in the future in hopes of changing the trajectory of his own life and timeline for the better.
How exactly do the two timelines work?
Unlike most time travel movies, The Tomorrow War doesn’t linger too long on its time travel logistics. It’s explained in the movie via a simple visualization: Time flows ever-forward, like a river. The scientists from 2051 have built two rafts on the river, one thirty years ahead of the other, and as far as time-traveling goes, they’re only able to hop between the two rafts.
While Muri and Dan do eventually engineer a toxin that could eliminate the aliens completely, they’re too late to save the world in 2051—the creatures have overtaken the base and all is lost. However, Muri’s plan is to send Dan back with a vial of the toxin in the hopes that he can save the world in the present day, effectively creating a new timeline at the expense of hers.
How does Dan save the world?
Upon returning from the future, Dan gets to work on a plan to stop the aliens before they arrive. He knows that the first attack is in the summer of 2048 in Russia, and that they arrived seemingly out of nowhere—no sign of rockets or ships were ever discovered. Emmy suggests that perhaps they may have arrived before 2048.
Dan then enlists the help of his time-jumping comrades Charlie (Sam Richardson) and Dorian (Edwin Hodge) to find out if the aliens may already be somewhere on Earth in their time, which would give them the opportunity to inject them with the toxin decades before they wreak havoc. Upon examining an alien claw Dorian kept as a “souvenir,” Charlie discovers particles of volcanic ash. One of Dan’s students helps them figure out that the ash is likely from the Millennium Eruption on the border of China and Korea, which happened in 946 AD, which means that the aliens had been here all along.