势在必行

势 = 力 + 执 : The space station's scientists want to conduct an experiment in the space station's bathroom (Ø4) to find the momentum (势) and potential energy (势) of a pendulum in zero gravity. Everything is already set up, they just have to push the power button (力)—but Sherlock Holmes (sh) insists on seeing their license (执) first before he allows it.

在 = 一 + 亻 + 土 : Zorro (z) and Rosa Luxemburg (亻) exist (在) in the airplane's bathroom (ai4). Rosa is in the middle of balancing a flute (一) on her head and on a pile of clay (土). Zorro uses his rapier to inscribe "Z" into the flute, so as to mark that the pronunciation of "在" begins with "z".

必 = 丿 + 心 : Mnemonic symbol: 忽必烈 [Hū​bì​liè​], Kublai Khan. Certainly (必) Kublai Khan (必) has to keep face, but he cannot hold it any longer. In the space station's bathroom (Ø4), He absolutely has to (必) tell Brunhilde (bi) his love for her strong character and offers her his heart (心). Brunhilde is a bit embarrassed because all she has to offer in return is a banana (丿). Kublai Khan feels like his heart has been crossed out (the 丿 stroke is crossing out 心 in 必).

行 = 亍 + 彳 : Mnemonic symbol from "to travel": seven-league boots.

Xi Shi (xi) is tired of walking with a black sneaker on her left foot (彳) and a full armor boot on her right foot (亍). Fortunately she finds seven-league boots (行) in the engine's kitchen ((e)ng2).