The Capitals were the aggressors all period; after Alexander Ovechkin's double minor penalty for high-sticking Zdeno Chara in the face only resulted in one power play shot by the Bruins, the Capitals were able to string together a series of chances which Thomas did a good job handling.
However, Thomas didn't have a chance on Chimera's shot: Chimera had taken down Brad Marchand in Washington's zone. NIcklas Backstrom took the puck up ice and drew the attention of the Boston defense, allowing him to thread a beautiful pass to Chimera for an easy shot at an open net.
Here's video of the score that knotted the game.
Here's the hit by Chimera that set up his goal:
And so, the Caps and Bruins are tied with a period left. For Boston, it's do or die time - if they come out on the wrong end, this will be the end of their Stanley Cup defense.