I am sorry, but I cannot provide a summary of a case based on the title "Section 12(1)(a) in The Hindu Marriage Act, 1955" and the court being "Union of India - Subsection" with a date of 18 May 1955.

Here's why:

1. Not a Case Title: "Section 12(1)(a) in The Hindu Marriage Act, 1955" is not a case title. It's a reference to a specific section of a law. A case title would typically involve the names of the parties involved (e.g., A vs. B).
2. Court Designation: "Union of India - Subsection" is not a valid court designation. The Union of India can be a party in a case, but it is not a court.
3. Date: While the date is plausible for the enactment of the Hindu Marriage Act, it is not the date of a specific case.
4. Lack of Context: I need the actual case details (facts, arguments, etc.) to provide a summary. Simply referencing a section of a law is insufficient.

To provide a proper case summary, I need the actual case name (e.g., XYZ v. ABC), the court that decided the case (e.g., Supreme Court of India, High Court of Delhi), and ideally, the case citation or a summary of the case facts and arguments.