When a well-respected mathematician is found dead, Holmes and Watson set out to solve a murder and the purpose of the highly guarded equation he was trying to decipher. Meanwhile, Watson is conflicted when the son of a former patient approaches her for a loan.

Sherlock helps a math genius from his network of 'Irregulars' experts after he stumbles upon a body while participating in a math puzzle competition. Joan attempts to extend an olive branch after she learns about Kitty’s past.

Holmes and Watson race to locate a missing woman when Harlan Emple, Holmes’ friend and “Irregulars” mathematics expert, becomes the prime suspect in her disappearance.