。
The lovers kissed.
Literal
Lovers [topic-は] kiss [object-を] did.
As short as Japanese gets. キス is a katakana loanword from English; する nominalizes any noun-action into a verb (キスをする = 'to kiss,' literally 'do a kiss'). The を can be dropped in casual speech, giving キスする — both are common. The 達 suffix on 恋人 marks plurality optionally, here important to fix the meaning as more than one person.