They both had a busy day doing intensive 3DMG training, almost leisurely soaring through forests together for hours on end to maintain their stamina and build core strength. It's not out of the ordinary when they come back from such an exercise with aching muscles, stinking of sweat that drenches their uniforms. Normally, they'd bathe separately, but when they arrive at their shared home, they're so exhausted that they decide to skip fighting over who gets to bathe first — and as such go to bed first — altogether and to just bathe together to save time and cuddle up in bed as soon as they're done.
It's not quite as quick as they hoped it'd be. Both take advantage of the private moment of downtime, Armin with his head resting against Bertholdt's chest as his eyes are closed, lying slouched against Bertholdt's frame — opting to sit like this to allow Bertholdt's legs to comfortably rest in the tub — and Bertholdt with his arms loosely wrapped around Armin's torso and with his head leaning forward, chin supported by the top of Armin's skull.
It's Bertholdt who finally breaks the silence, suggesting that they finish bathing while the water's still warm lest they risk falling asleep in the tub and having to climb out of cold water without having gotten properly clean, something neither of them is particularly in the mood for. He pours water over Armin's head for him — giving him the luxury of relaxing while the work's done for him — before applying shampoo to his hair and scalp, rinsing it out, and getting to work massaging the tenseness out of his shoulders, back, arms, buttocks, thighs, calves, and feet while he applies soap to those areas, convincing him halfway through to sit on the side of the tub facing him to better reach his lower body. The two have long grown accustomed to seeing each other bare, but that doesn't stop Armin from teasing Bertholdt about wanting to see him sit in front of him like that so badly, and maybe, that causes Bertholdt to retort with actions, not words, that perhaps validate Armin's provocations.
Bertholdt assures him he can leave the tub and work on drying himself off after he's rinsed the soap off again, but the smaller male refuses, insisting he must return the favour with renewed gusto. Bertholdt instantly feels like he could've done better when Armin plants kisses all over Bertholdt's body as he runs his fingers through the brunet's hair, but Armin is effective in interrupting all apologies locking lips with the aforementioned apologies' source.
By the end, when they've both gotten just dry enough not to crawl into bed dripping and special care has been put into brushing through Armin's hair, they decide against putting on pyjamas, making their way straight to bed to allow their sore bodies to heal over the night. Armin's bed-dried hair is the worst in the morning.