Somebody will ask. Usually it is the second or third question after your name, and it is asked warmly rather than suspiciously. The reason it lands awkwardly is almost never the question. It is that two people are answering it for the first time, in front of an audience, without having compared notes.
Fix that before you leave the house. Decide on one sentence, write it down, and send it to your concierge so your companion has it in their briefing. That is the whole technique.
Short beats clever
The best answers are boring. "Through friends" has closed this conversation for a century and continues to. "We met at a work thing last spring" is equally serviceable and slightly more specific, which makes it easier for both of you to remember under pressure.
What fails is the elaborate story. A detailed account invites follow-up questions, and follow-up questions are where two people who have not rehearsed start contradicting each other. If your answer needs a second sentence, it is too long.
Pick a season, not a date
Vague timing is your friend. "Last spring" cannot be checked against a calendar, whereas "the fourteenth of March" can and occasionally will be. People remember specifics and then ask about them later.
- Agree the sentence, not the story
- Choose a season rather than a month or a date
- Pick a setting neither of you will have to describe in detail
- Send it to your concierge so it reaches your companion in writing
Decide who answers
This is the part people skip. If you both start talking, the pause afterwards does more damage than any answer could. Agree that you take it, and that your companion adds a detail only if asked directly.
The question is not a test. It is small talk. Treat it as small talk and it stays small.
When honesty is the easier route
Some clients tell their table exactly what the arrangement is, and it goes fine. This is more common than it used to be, particularly at industry events where a plus-one is understood as a practical arrangement rather than a romantic one. If that is the version you want, tell us and your companion will match it without hesitation.
What matters is not which version you choose. It is that both of you are working from the same one before the first drink.