Christmas episode. Has about as much heft as the phrase "Christmas episode" suggests. Lots of monochromatic snow imagery, lots of Christmas kitsch imagery, all of which I'm an easy mark for. Dropped hints about who has a crush on whom; empty-calorie human interest subplottery. Herogirl's kid sister drops some hints about how Herogirl once bought a Christmas gift for a boy she loved but she never gave it to him. Later, it's cold and Heroboy doesn't have any mittens, so Herogirl gives Heroboy some mittens. HMMMM.
Also Ponytail Scientist Guy finds a big pretty crystal stuck in the wings of the Rahxe-Robot and doesn't stop to consider that it might be dangerous. Nope, he just puts it on a necklace and gives it to Blonde Scientist Lady. No one notices when it begins growing, as a spooky phantom Mu Pilot (all the Mu monsters have human pilots operating them be remote control) appears in the window behind her, watching, waiting...
In the second act all the pretty snow turns into freezing imagery as a wintry Mu grows from the crystal necklace and absorbs Blond Scientist Lady in its ice, with funky visual stylings. Heroboy in his Rahxephon tries to save her; he can't hit the Mu because BSL is a human shield. Mu freezes Rahxephon with wintry ice magic as BSL gives a manic monologue about how unloved she is and how cold (geddit?) Heroboy is. And she weeps blood.
Then Yellow-Ribbon Mysterious Girl manifests inside the Rahxephon and gives Heroboy a little cuddle, which warms him up and thaws the ice. Hot sphere of sun shimmers behind Rahxephon as it whips up a force sword somehow and carves BSL out of her Mu. Day saved. By cuddles. My kind of Deus Ex.
Coda: Christmas party. Herogirl's kid sister notices Heroboy's gloves and mentions that they're the gift Herogirl bought for her long lost love. HMMMMM.