Under what condition can a third party defendant assert a claim against the plaintiff?

The third-party defendant can assert a claim against the plaintiff if it arises from the same transaction or occurrence as the plaintiff's claim. This principle is rooted in the concept of relatedness in litigation, where claims that share a common nucleus of operative fact may be heard together to promote judicial efficiency and avoid inconsistent results.

When the third-party claim is connected to the original plaintiff's claim, it ensures that all related disputes are resolved in a single proceeding, thereby minimizing the need for multiple trials and conserving judicial resources. This is particularly relevant in scenarios where the third-party defendant believes that the plaintiff may share some fault or liability regarding the same incident or transaction that gave rise to the initial claim.

In contrast, claims that are independent and not related to the plaintiff's claim would not meet the criteria for assertability within the same action, limiting the opportunity for a third-party defendant to create further litigation that is unrelated to the original issue. Similarly, scenarios where both parties agree or where the plaintiff has admitted liability do not establish the necessary legal foundation for the third party to assert their claim, as they lack the essential connection to the underlying dispute.