。
I went to Sendai and came straight back without stopping.
Literal
Sendai [to-へ] go-[and-て] dragonfly-return [by/in-で] came-back-[polite-ました].
とんぼ返り ('dragonfly's return') is a charming idiom — a quick round trip where you return immediately after arriving, as though only touching down briefly like a dragonfly at a pond. The word can also mean a literal somersault. The particle で marks manner ('by way of / in the manner of'), and 戻ってくる is 戻る + ~てくる, where ~てくる adds motion back toward the speaker's location.