The forensics buddy cop drama “Rizzoli and Isles” (8 p.m., TNT) enters its fourth season in domestic mode. The show opens with detective Jane Rizzoli (Angie Harmon) and her best friend, Dr. Maura Isles (Sasha Alexander), dressed in their pajamas in the kitchen, caring for an infant like doting married parents. Only the baby belongs (or so we suppose) to Rizzoli’s not-so-organized brother, Tommy (Colin Egglesfield), and his on-again-off-again girlfriend, who just abandoned the baby on Rizzoli and Isles’ doorstep. Whatever the circumstances, Rizzoli and Isles seem to be eager to raise the child as a family, or as part of some extended unit.

For the uninitiated, Rizzoli, played by the impossibly good-looking Harmon, is the tomboy of the couple — or rather, the pair. Isles is the intellectually snobbish clotheshorse, a total girly-girl. Much like Dr. Temperance Brennan on “Bones,” Isles projects an academic detachment from ordinary life so she can better master mortuary science. When not discussing domestic issues, the banter between the two women often revolves around corpses or the gross stuff found inside them. Isles offers up her findings with a combination of pride and icy detachment; Rizzoli responds with wisecracks and an air of blue-collar practicality.

In tonight season’s premiere, a cadaver betrays signs of a possible contagion, sparking a code red. Not to give too much away here, but a citywide plague is averted and a major crime is solved, all before we return to the baby and scenes of domestic tranquility.

The ambiguity of the Rizzoli and Isles’ relationship is clearly intended and skillfully played out. They’re like the Bert and Ernie of forensic detectives. The decision to keep things between them deliberately unlabeled is very much like the show itself, and perhaps its audience. It’s decidedly, even audaciously, old school.

• Tonight’s holiday specials include one of the very best of the season, the 1966 animated fantasy “Dr. Seuss’ How the Grinch Stole Christmas” (7 p.m., ABC), narrated by Boris Karloff.

Tonight’s other highlights

• A teen patient puts Mindy on the spot on “The Mindy Project” (8:30 p.m., Fox).

• The summer Olympics helped promote “Go On” (8 p.m., NBC), and now it returns the favor. Look for a cameo from beach volleyball star Misty May-Treanor.

• Savino’s ex lands a singing job on “Vegas” (9 p.m., CBS).

• Julia chafes at her stay-at-home status on “Parenthood” (9 p.m., NBC).