A museum curator finds an ancient dagger tucked inside a crate as Forever Season 1 Episode 22 begins. She ends up dead under a tunnel and Lucas shows up to help Jo and Hanson solve the case since Henry is nowhere to be found. He and Abe are busy burying Abigail.
Abe advises Henry to think of the good times he and Abigail shared together which prompts Henry to remember the vow they swore to protect each other. Henry swears vengeance on Adam for taking Abigail's life.
When Henry and Jo examine the victim's office, he learns that the woman had discovered a dagger which could prove useful in killing Adam. Henry clues Abe in to Adam's theory about the weapons which can kill them and Abe locks the pistol in a safe so Henry doesn't have access to it.
The dagger for which Blair Dryden was killed is believed to be linked to the death of Julius Caesar. Blair's boyfriend admits to getting his best man to mug his fiance in the park in order to sell the knife and tells them where to find his friend. Adam has already been there but the dagger was already gone. Adam warns Henry to stay away.
Abe tells Henry not to let rage cloud his judgment when it comes to handling Adam. Henry decides to steer the investigation away from the dagger, but Lucas blows his plan by finding a connection between the two murders. At the museum, Henry discovers that a security guard had something to do with the deaths.
When the security guard appears to have a heart attack while in custody, Jo suspects Henry had something to do with it and learns that he's on a hunt for the dagger. The guard sends Henry to the collector they spoke with earlier in the investigation and Henry learns that the dagger was used by Nazi doctor Josef Mengele. Mengele kept a diary about his experiments on Adam and the collector is fascinated with the story. Jo arrives and takes the dagger from Henry.
Henry enters his basement lab to find Abe guarding the safe with a gun. He realizes that even if it means Jo will hate him, he has to get the knife in order to protect her. Lucas has helped him retrieve the weapon from evidence lockup and Henry heads into the subway to deliver the knife to Adam. Jo is on his tail but he loses her when he goes under the tunnels.
Henry tosses the knife to Adam and tries to walk away but Adam has his gun. He fires a shot which draws Jo into the tunnels and then shoots Henry so that he'll either die or have his secret discovered. Henry expires but not before stabbing Adam in the neck with a syringe. Jo finds only Henry's pocket watch in the tunnels and Abe is waiting for Henry to resurface.
Jo returns Henry's pocket watch and a picture she found of him, Abigail, and Abraham in the 1940s. She asks for an explanation and Abe urges Henry to tell her the story.