>Hi everybody,
>
>For my son's 9th birtday my husband bought him a Ninja-GO Lego (that's what he asked). It looks like at least one detail is missing from the set.
>
>So, what should we do in such situation?
>
>Thanks.

Take it back to the store from which you bought it. Depending on the store you may or may not need the original receipt. Explain that the set is incomplete and ask for a replacement. I'm reasonably sure that almost any reputable store will allow the exchange without question.

If they don't then follow Craig's advice.